Fig. 1.

The tendency for animals from the same burrow system to have the same disease status is illustrated here by plotting the observed distributions of susceptible, recovered and infectious animals captured from the same burrow system on a de Finetti diagram [(b) and (e)]. The size of each point on the triangle represents the number of burrow systems having that particular distribution. Burrow systems with distributions lying on the vertices of the triangle represent those that are wholly susceptible, infectious or recovered. The mean distribution of susceptible, infectious and recovered animals for the population as a whole is denoted by an asterisk (*). At site 1 a total of 472 gerbils were sampled from 99 burrow systems and at site 2 it was 186 from 45. Simulation (where the sampling was mimicked but the population average used to generate the numbers of susceptible, infectious and recovered gerbils in each burrow system) was used to obtain frequency distributions [(a) and (d)] for the variance in the within-burrow-system distributions. This variance is that expected from the demographic stochasticity associated with the low numbers of gerbils trapped from each burrow system. The histograms are based on 999 simulations. One set of simulated distributions for each site are shown on a second pair of de Finetti diagrams [(c) and (f)].